Optical communications systems utilize optical signals to transmit information among various items of optical equipment that are coupled to the systems. The systems utilize optical fiber cables for transmitting the carrier waves from one item of equipment to another. For example, an optical communications system may comprise a computer central processing unit (CPU), a workstation, a peripheral, such as a printer, each of which is equipped with optical transmitting and receiving devices, and optical fiber cables linked among the CPU, the workstation and printer.
Each item of optical equipment is coupled to the optical fiber cables by means of an optical connector to allow a means of disconnecting the equipment from the optical fiber cables. Such systems may utilize two optical fibers, one for receiving optical signals from an item of optical equipment, and another for sending optical signals. Each optical transmitter has an optical emitter for sending the signals, and each optical receiver has an optical detector for receiving the signals.
Testing of such items of optical equipment is a necessity to assure proper design. In testing, the test conditions must accurately simulate the anticipated operating environment. In operation, systems of optical fiber waveguides experience attenuation, which is loss of the transmitted optical power. Attenuation for fiber is specified in decibels per kilometer(dB/km).
Systems utilizing optical fiber cables and other components are specified in terms of the maximum optical attenuation that can occur between the transmitting and receiving devices, while still providing information transfer with substantially no errors. Typically, testing of such systems is done by simulation whereby the emitter and detector of the item of optical equipment to be tested is connected to a device that simulates the optical system, and the operation of the item is tested as though the item were coupled into the system itself and not the testing device.
First simulators were devices capable of generating special test signals. Testing was performed externally on the item of equipment being tested. Recently, optical equipment has been designed with internal testing capabilities. With self-testing, the expense of specialized testing equipment and associated testing procedures has been substantially reduced. In place of long lengths of cabling to simulate actual operations and in place of simulators that are devices that produce complex signals or measurements, are simplified simulators comprising internal attenuating devices such as the simplified loop-back attenuator. It is anticipated that such simulation will be used primarily as a simplified and inexpensive means of diagnosing and localizing failures in complex systems of installed equipment.
The present invention relates to simulators which are loop-back attenuators, defined as simulators providing a communication signal path that forms a loop from the emitter to a detector of the same item of optical equipment such that optical signals transmitted from the item under test are looped back to the same item and internally transmitted among its component parts. Consequently, communications from a transmitter to a receiver within the unit of equipment can be accomplished without operation of other units of equipment. Functionality of the optical transmitter and receiver, as well as all electronic circuitry used to generate the required optical signals, can be quickly determined. Simulators which are loop-back attenuators purposely simulate a loss of signal intensity expected of a communications system in which the item may be installed for "on-line" operation. Vastagh, U.S. Pat. No. 4,736,100, discloses a known loop-back attenuator involving an optical fiber cable formed in a loop and having ends of the fiber connected with alignment ferrules. The loop is installed in an alignment fixture that will align the ends of the loop with the emitter and detector of the item to be tested.
This known loop-back attenuator suffers from disadvantages, mainly that of accurately duplicating the amount of attenuation in the operations system so that the testing device creates an environment approximating the operation of the actual system for meaningful test results. Additionally, results can significantly vary from one type of transmitting or receiving device to another since there is no coupling and confinement mechanism similar to that which the optical emission will encounter in actual use.
Objects of the present invention include providing a simulator in the nature of a loop-back attenuator that, in a compact device, is capable of reproducing the total attenuation of a substantially larger cable network. Other objects include providing a device capable of sufficiently attenuating optical power between emitter and detector of a transceiver or the like, to prevent saturation of the detector, and providing a device which easily and accurately may be controllably altered to match the particular amount of attenuation desired to simulate actual environmental operating conditions or to meet manufacturer's standards.
Another problem is that devices of different manufacturers, and even, indeed, the same device of the same manufacturer, have emitters that put out differing optical power. An objective in this respect, is to provide a device which may easily be altered to accommodate the differing optical power outputs of devices and the attenuation characteristics to be expected in the operating systems. Hence, another objective of the present invention is providing an optical simulator which is a loop-back attenuator usable on a variety of combinations of emitter and detector elements of a transceiver, which will approximate operating environments and which will effectively reduce power output so that the detector will not be saturated, or so that the detector receives a specific optical power level.
These objects are achieved by utilizing the mechanism of the present invention in combination with known mechanisms to create attenuation in the fiber optic loop. Such known mechanisms include numerical aperture mismatch between the fiber end of the emitter or detector, core diameter mismatch of different fibers making up the loop, and filtering optical power by use of a filter in the form of a glass or film. Such known mechanisms and simulators utilizing such mechanisms are useful in approximating a targeted attenuation, but often do not provide sufficiently exact values of attenuation, and are not generally variable, once produced. Useful would be a device capable of adjusting the attenuation achieved by the approximating mechanisms of core diameter mismatch, use of filters or the like.
By the present invention, attenuation in the loop is increased and fine tuned by controllably inducing bending in the optical fiber that forms the loop of the attenuator as described hereinafter.
The increase in attenuation brought about by bending is based upon the structure of the optical fiber which typically consists of two concentric regions; the core, which has a refractive index higher than the outer region, the cladding. Light injected into the core and striking core-to-cladding interface at an angle greater than the critical is reflected back into the core, or, for graded index fibers, is continually refracted in such a manner as to confine the light to approximately the core region. In both types of fibers, attenuation is changed by bending the fiber, and such change depends on the bend radius, among other factors.